From 26769f96e6231095f6b1cc3090c903280d44bb57 Mon Sep 17 00:00:00 2001 From: Marcelo Tosatti Date: Tue, 16 Jun 2020 08:47:41 -0300 Subject: [PATCH] KVM: x86: allow TSC to differ by NTP correction bounds without TSC scaling The Linux TSC calibration procedure is subject to small variations (its common to see +-1 kHz difference between reboots on a given CPU, for example). So migrating a guest between two hosts with identical processor can fail, in case of a small variation in calibrated TSC between them. Without TSC scaling, the current kernel interface will either return an error (if user_tsc_khz <= tsc_khz) or enable TSC catchup mode. This change enables the following TSC tolerance check to accept KVM_SET_TSC_KHZ within tsc_tolerance_ppm (which is 250ppm by default). /* * Compute the variation in TSC rate which is acceptable * within the range of tolerance and decide if the * rate being applied is within that bounds of the hardware * rate. If so, no scaling or compensation need be done. */ thresh_lo = adjust_tsc_khz(tsc_khz, -tsc_tolerance_ppm); thresh_hi = adjust_tsc_khz(tsc_khz, tsc_tolerance_ppm); if (user_tsc_khz < thresh_lo || user_tsc_khz > thresh_hi) { pr_debug("kvm: requested TSC rate %u falls outside tolerance [%u,%u]\n", user_tsc_khz, thresh_lo, thresh_hi); use_scaling = 1; } NTP daemon in the guest can correct this difference (NTP can correct upto 500ppm). Signed-off-by: Marcelo Tosatti Message-Id: <20200616114741.GA298183@fuller.cnet> Signed-off-by: Paolo Bonzini --- arch/x86/kvm/x86.c | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-) diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c index 29d9b078ce69..3b92db412335 100644 --- a/arch/x86/kvm/x86.c +++ b/arch/x86/kvm/x86.c @@ -4603,7 +4603,8 @@ long kvm_arch_vcpu_ioctl(struct file *filp, r = -EINVAL; user_tsc_khz = (u32)arg; - if (user_tsc_khz >= kvm_max_guest_tsc_khz) + if (kvm_has_tsc_control && + user_tsc_khz >= kvm_max_guest_tsc_khz) goto out; if (user_tsc_khz == 0)